In recent years systems have been developed for performing authentication of individuals based on biological information instead of on the PIN numbers or ID cards that have been used in the past in fields that require individual authentication, such as control of entry/exit into computer rooms or critical equipment rooms, control of access to computer terminals or financial terminals in banks, and the like.
Personal authentication systems that use biological information are of interest because they do not require memorization or possession of objects, and have a low risk of theft. While fingerprints, irises, faces, handprints, signatures, and the like are used in personal authentication using biological information, of these, personal authentication using the face is attracting attention as having excellent convenience and acceptance.
For example, in a personal authentication system used in entry/exit control of a building, or the like, a picture is taken of the face of the individual who will attempt to enter the building, and that picture is used as a reference image, where that reference image is compared to an image of the face of the individual that has been registered in advance (a registered image), to allow the individual to enter the building if the reference image and the registered image match.
The present applicant, in the process of comparing the reference image and the registered image, uses, for the respective reference image and registered image, a raw image as a high-resolution image that has the highest resolution, and creates a plurality of hierarchical images with different resolution levels with the high-resolution image at the highest level, to perform a search for corresponding points using a phase-only correlation method (POC), moving from the low-resolution images of these hierarchical images towards the high-resolution images (referencing, for example, Japanese Unexamined Patent Application Publication 2008-209275 (“JP '275”) and Japanese Unexamined Patent Application Publication 2008-123141 (“JP '141”), to perform the comparison of the reference image and the registered image based on the phase-only correlation values of the corresponding regions after searching. In the below, the method for performing a comparison after searching for corresponding points using hierarchical images shall be termed “hierarchical POC.”
In this hierarchical POC, with, for example, the raw image (a 128×128-pixel image) as the high-resolution image for the reference image, a 64×64-pixel image (a first low-resolution image) is created on the level under the high-resolution image, and a 32×32-pixel image (a second low-resolution image) is created on the level under the first low-resolution image. Moreover, as with the case for the reference image, for the registered image as well, the raw image (a 128×128-pixel image) as the high-resolution image, a 64×64-pixel image (a first low-resolution image) is created on the level under the high-resolution image, and a 32×32-pixel image (a second low-resolution image) is created on the level under the first low-resolution image. In the hierarchical images, including the raw images, the hierarchical level of the high-resolution images is defined as the zeroeth level, the hierarchical level of the first low-resolution image is defined as the first level, and the hierarchical level of the second low-resolution image is defined as the second level.
Moreover, m reference points are established in the x direction and n reference points are established in the y direction for the raw image of the registered image, and, for each reference point established for the raw image of the registered image, a search window (a local region) is established from the hierarchical image at the lowest level (the second level), to start the search for the corresponding region in the reference image corresponding to that local region. The search for the corresponding region is performed using POC comparison, and when the search for the corresponding region for the applicable hierarchical image is completed, the search results for the applicable hierarchical image are reflected into the search on the next hierarchical level (the first level), to perform searching for the corresponding regions for hierarchical images of sequentially higher resolutions, and the search for corresponding regions is repeated until arriving at the hierarchical image at the highest level (the zeroeth level).
Additionally, in the hierarchical image at the highest level (the zeroeth level), if a corresponding region is found, then a phase-only correlation value is calculated for that corresponding region. Thereafter, searching is performed in the same manner, searching for the corresponding region of the reference image for each local region of the registered image, starting with the low-resolution image, moving towards the high-resolution image, and calculating the phase-only correlation value for the corresponding region after searching, and then comparing the reference image and the registered image based on the calculated phase-only correlation values.
Note that POC is an algorithm for calculating the correlation between two images by performing mathematical processes on the original image data and on the data of the image to be compared thereto through a Fourier transform, to analyze into amplitudes (shading data) and phases (image contour data), and then using only the phase information to calculate the correlation between the two images, and has the distinctive features of being robust to external noises, to enable highly precise calculation results.
POC is explained in detail in JP '275 and JP '141, referenced above, and also in Japanese Unexamined Patent Application Publication H10-63847, and so forth. In POC, a two-dimensional discrete Fourier transform is performed on one of the images to produce a Fourier image. Following this, a Fourier image of the other image, produced through performing the same process as for the earlier Fourier image, is combined, and after performing a process to set the amplitudes of the combined Fourier images to 1, a two-dimensional discrete Fourier transform is performed to produce the correlation data. This correlation data (the POC function) has the amplitudes in the frequency space set to 1, so uses only the phases; however, fundamentally this can be considered to be data wherein one image is convolved with the other image, to express the correlation between the one image and the other image.
However, while it is possible to perform the comparison of the reference image and the registered image with high precision in the comparison using the hierarchical POC described above, the processing overhead is large when applied to 1:N comparisons wherein the reference image is compared to a plurality of registered images, and thus there is a problem in that this causes the speed of comparison to suffer.
The present invention is to solve problems such as described above, and the object thereof is to provide a comparing device able to provide both high precision and high-speed processing.